In general, olivine structure lithium metal phosphate (LMP) used as a positive electrode active material for a lithium secondary battery is being manufactured by the following two methods.
A first method is to manufacture lithium metal phosphate by using a divalent iron raw material. However, the divalent iron has a problem in that the divalent iron is less stable than trivalent iron, more expensive than a trivalent iron raw material, and easily oxidized, and thus, it is difficult to manage the raw material, and also has a problem in that the specific weight of a volatile material of respective raw materials is high so that the specific weight of lithium metal phosphate synthesized after calcination is low, thereby making battery characteristics deteriorate.
A second method is to manufacture lithium metal phosphate by using amorphous iron phosphate salt hydrate (Amorphous FePO4.2H2O) or amorphous iron phosphate salt anhydride (Amorphous FePO4) as a precursor. However, these methods use, as a precursor, an amorphous iron phosphate salt in which iron and phosphate salt atoms are randomly mixed, and thus, have a problem in that it is required to maintain high temperature during the calcination, and the growth of particles and an increase in process costs are incurred during the high temperature synthesis process.
Thus, as a new synthesis method which may solve the aforementioned problems, the present inventors disclosed in the following previous patents that when lithium transition metal phosphate is synthesized by using, as a precursor, crystalline iron phosphate salt (FP) having a (meta)strengite structure or metal-doped crystalline iron phosphate salt (MFP) having a (meta)strengite structure, due to the structural similarity of precursors and products, high-crystalline lithium metal phosphate may be synthesized at a temperature which is much lower than a temperature at which the amorphous iron phosphate salt hydrate is used as a precursor, and the growth of particles may be controlled at a low calcination temperature, thereby reducing process costs.
1. Korean Patent Application No. 10-2011-0136537
2. Korean Patent Application No. 10-2011-0136536
However, the two patents suggest a method for manufacturing LMP by synthesizing and drying crystalline iron phosphate salt ((M)FP), and then solid-phase or wet mixing the crystalline iron phosphate salt ((M)FP) with a lithium source and a carbon source, and then drying the mixture. In this case, primary particles of the (M)FP are nanoparticles having a size of 100 nm or less, and form a strong aggregate during the drying step. Therefore, since it is required to perform a process of disentangling aggregation of the (M)FP during the mixing of the (M)FP with the lithium source and the carbon source in order to synthesize LFP, or the aggregate may hinder materials from being homogeneously mixed, efforts to enhance the efficiency in the process of manufacturing lithium metal phosphate have been continuously made.